2N3904 Transistors Features, Pin configuration, and Frequency Testing

2N3904 Transistors

2N3904 Transistors Introduction

2N3904 transistors are a type of bipolar junction transistor (BJT). They are commonly used in electronic circuit applications as a switch or amplifier. The 2N3904 consists of three pins: the collector, base, and emitter. It is a general-purpose NPN transistor that can be used for switching and amplifying applications.


  • Current – Collector (Ic) (Max): 200mA
  • Voltage – Collector Emitter Breakdown (Max): 40V
  • Vce saturation (maximum) under Ib, Ic conditions: 500mV @ 50mA, 500mA
  • Minimum DC Current Gain (hFE) at Ic, Vce: 100 @ 150mA, 10V
  • Power – Max: 800mW
  • Frequency – Conversion: 100MHz
  • Package/Case: TO-39-3, TO-205AD

Pin configuration for 2N3904 transistors

The Transistor consists of three pins.

  1. The first pin is the emitter, which allows electric current to flow out of the Transistor.
  2. The second pin is known as the base and is useful in adjusting the biasing of the Transistor.
  3. The third pin is the collector and electric current will enter the Transistor through this pin.

Characteristic Frequency Testing of Transistor

The transistor characteristic frequency can be measured by using Tektronix oscilloscopes, signal generators, and Keithley source meter products for AC parameter testing of transistor devices. As the actual operating frequency of the transistor is much higher than the low-frequency current gain cutoff frequency fβ, the AC current gain is inversely proportional to the operating frequency, and the “gain-bandwidth product” of the transistor is constant, approximately equal to the working frequency when the modulus of the common-emitter current gain is 1. The measurement of the characteristic frequency of bipolar transistors is to couple a high-frequency small AC input signal of a specific frequency to the base through a capacitor, change the DC bias conditions of the common-emitter configuration transistor, and thereby change the AC current gain to study the relationship between the characteristic frequency of the transistor and the DC operating point.

Experimental equipment: Tektronix MSO34-BW500 oscilloscope, Tektronix AFG31251 signal generator, digital multimeter, two digital source meters, transistor DC/AC parameter comprehensive experimental board.

Power supply for the experimental board

This experiment uses the 2N3904 NPN transistor, and two source meters are used to provide IB and Vce. The pin definitions of the 2N3904 transistor in the TO-92 package are shown in the following figure:

2N3904 transistor pin definitions

The 2N3904 on the fixture should be installed with the plane facing to the right, and the three pins from top to bottom are C/B/E.

Setting and testing the static operating point of the transistor

The working range of the 2N3904 is shown in the following figure:

Working range of 2N3904 transistor

To ensure that the transistor operates in the amplification region, the DC operating point of the transistor is set to IC=1mA. When no AC signal is connected, the static working circuit diagram of the transistor is shown in the following figure:

Static working circuit diagram of 2N3904 transistor

Adjust the IB output of SMU1 and observe that the IC current value of SMU2 is approximately 1mA to ensure that the transistor operates in the amplification region. The IB is approximately 2.8uA, and the BE voltage VBE is 0.636V as measured by a multimeter.

Testing the h-parameters of the transistor: hie and hfe


With a reasonable setting of the static operating point and an AC small signal input, the transistor can be equivalent to a linear two-port circuit, represented by AC components of current and voltage. Where Ib and Vbe are the input variables of the transistor, and Ic and Vce are the output variables. The h-parameters of the transistor reflect the small-signal AC characteristics of the transistor under certain fixed static conditions.

Connect the signal generator output to the BNC interface on the left side of the experimental board’s AC IN, and connect the oscilloscope Channel 1 to the BNC interface on the right side of the experimental board’s AC OUT.

Set the signal source output to a 1 kHz sine wave, adjust the signal source output signal amplitude, and use oscilloscope channel 2 to test the voltage waveform between the two terminals of R1 (connect the banana head interface marked as Input). Calculate the effective value of Ib current so that Ib is approximately equal to 0.5 uA.

Set the signal source output to a 1 kHz sine wave and change the output amplitude. When the effective value of the voltage between the two terminals of R1 measured by the oscilloscope is 50 mVrms, and since R1 = 100 kohms, Ib is approximately equal to 0.5 uA.

“hie” is the input resistance when the output is short-circuited and reflects the ability of the base voltage to control the base current with output voltage Vce unchanged.

ℎ𝑖𝑒 = 𝑣𝑏𝑒𝑖𝑏 = 𝑣𝑏𝑒𝑣𝑖𝑛𝑝𝑢𝑡∗ 𝑅1

Under the condition that the above test conditions remain unchanged, the effective value of Vbe measured by 2 channels of the oscilloscope is 5.7mVrms.

hie = Vbe/ib = Vbe/Vib * R1 = 5.7/50 * 100K = 11400, where Vbe is the effective value of the Vbe and Vib is the effective value of the input voltage under the above test conditions.


“hfe” is the current amplification factor when the output is short-circuited and reflects the ability of the base current ib to control the collector current ic, that is, the current amplification ability of the transistor.

ℎ𝑓𝑒 = 𝑖𝑐𝑖𝑏 , 𝑤ℎ𝑒𝑟𝑒 𝑖𝑐 = 𝑣𝑜𝑢𝑡𝑝𝑢𝑡/𝑅2

hfe = ic / ib, where ic = voutput / R2 = 17.3 mV / 100 ohms = 0.173 mA. voutput is the effective value of the output voltage and R2 is the resistance of the load connected between the collector and the power supply.

Cut-off frequency fβ and characteristic frequency fT

Measure the transistor’s cut-off frequency fβ and calculate its characteristic frequency fT using the “gain-bandwidth product” method.

Gradually increase the output frequency of the signal source from 1 kHz, and observe the amplitude of the AC OUT signal on the right side of the experimental board using the oscilloscope. When the output signal amplitude drops by 3 dB (peak-to-peak value drops by half), record the output frequency fβ of the signal source, indicating the cut-off frequency of the transistor at the current working point.

At 1 kHz, the AC OUT output peak-to-peak value is approximately 38 mV, and at 1.4 MHz, the AC OUT output peak-to-peak value is approximately 19.2 mV.

Calculate the characteristic frequency fT of the transistor using the gain-bandwidth product formula:

fT = hfe × fβ
fT = 228 * 1.4 = 319.2 MHz

where fβ is approximately equal to 1.4 MHz.

Verify the characteristic frequency fT of the transistor using a high-frequency signal source and a 500 MHz bandwidth oscilloscope. If the bandwidth of the oscilloscope and signal source is greater than 200 MHz, you can use the signal source to scan the input signal in the frequency range above DC-200MHz, and test the amplitude-frequency characteristics of the output signal (AC OUT) on the oscilloscope, and calculate the current magnification down to 1 manually find the characteristic frequency point fT. Verify that the eigenfrequency values calculated by the gain-bandwidth product method are accurate.

2N3904 Vs. 2N2222 Transistors

Here is a comparison of 2N3904 and 2N2222 transistors below:

Transistors TypeNPNNPN
Maximum Collector Current200 mA800 mA
Maximum Collector-Emitter Voltage40V40V
Maximum Collector-Base Voltage50V50V
Maximum Emitter-Base Voltage5V6V
Maximum Frequency300 MHz500 MHz
Package TypeTO-92TO-18, TO-92


– Amplifiers
– Switches
– Voltage Regulators
– Converters
– Timers
– Frequency Modulators
– Motor Drivers
– Signal Processing Circuits
– Audio Circuits
– Power Supply Circuits
– Comparators

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